Method and apparatus for monitoring diffusion pump back-streaming in the throat of said pump



Jan. 10, 1967 P. B. KENNEDY 3,297,872

METHOD AND APPARATUS FOR MONITORING DIFFUSION PUMP BACK-STREAMING IN THE THROAT OF SAID PUMP 5 Sheets-Sheet 1 Filed April 22, 1964 47 L T l G UE- 1 1 39 46 32 2: 59 63 Q 66 27 5 A6 I .251

,ziz a," INVENTOR.

PATRICK B. KENNEDY ATTORNEY,

Jan. 10, 1967- P. B. KENNEDY METHOD AND APPARATUS FOR MONITORING DIFFUSION PUMP BACK-STREAMING IN THE THROAT 0F SAID PUMP Filed April 22, 1964 3 Sheets-Sheet 2 INVENTOR, PATRICK B KENNEDY 123a FmD qIxw ATTORNEY.

Jan. 10, 1967 p, KEN DY 3,297,872

METHOD AND APPARATUS FOR MONITORING DIFFUSION PUMP BACK-STREAMING IN THE THROAT 0F SAID PUMP Filed April 22, 1964 3 Sheets-Sheet 3 INVENTOR. 3 PATRICK B. KENNEDY X M4 Wyn,

ATTORNEY.

United States Patent 3,297,872 METHUD AND APPARATUS FUR MONITORING DHFFIUSIGN PUlVriP BACK-STREAMING 1N THE THROAT @F SAID IUMP Patrick 13. Kennedy, El (:errito, Caliif assignor to the United States of America as represented by the United States Atomic Energy Commission Fiied Apr. 22, 1964, Ser. No. 351,922 13 Claims. (Cl. 250-65) The present invention relates to the monitoring of vacuum pumps and more particularly to a method and apparatus for precisely determining the extent and distribution of back-streaming vapor within an operating diffusion pump of the type which employs jets of mercury or oil vapor to entrain and withdraw the gas evacuated from a vessel. The invention described herein was made in the course of, or under, Contract W-7405-eng-48 with the Atomic Energy Commission.

Diffusion pumps of the type on which the present invention is operative generally comprise an upright cylindrical pump housing communicating with the vessel to be evacuated, a pool of mercury or oil contained within the housing, a heater for producing a vapor of this fluid under pressure, and one or more jet nozzles for channeling the pressurized vapor in a direction away from the vessel to be evacuated. Such jet nozzles are generally formed to produce a conical vapor jet directed against the walls of the pump housing, which vapor jet entrains gas molecules arriving from the evacuated vessel and subsequently condenses and returns to the pool. A small amount of this vapor generally drifts in the direction of the vessel and subsequently contaminates the vacuum therein. This effect, termed back-streaming, is of considerable significance as it is a principal factor which limits the degree of vacuum that can be obtained.

Back-streaming may be reduced to some extent by modifying certain design or operational features, for example by disposing refrigerated baflles or traps between the vapor jets and the vessel, or supplying extra heat to the system to minimize premature condensation of the vapor.

'lo efliciently design these and other modifications as well as to monitor the normal operation of the pump it is desirable to determine accurately the pattern of backstreaming in any given pump. Accordingly, it is the purpose of the present invention to provide a method and apparatus for precisely measuring the migration of the pumping fluid and for locating the sources of back-streaming fluid by determining the pattern of vapor flow in a cross-section of the pump throat.

Essentially, this is accomplished by providing a novel form of camera to obtain a pictorial representation of the back-streaming which occurs during pump operation. The camera is comprised of a thin, immaculately clean plate of a suitably adsorptive material which is momentarily interposed, through an air-tight housing, between the jet nozzles of the diffusion pump and the vessel being evacuated so that the back-streaming fluid becomes deposited on the surface of the plate toward the pump. As soon as a complete pattern ofback-streaming is obtained, generally in only a few seconds, this plate is removed from the housing without disturbing the evacuating operation and then exposed to a treatment that will make the fluid droplets deposited thereon clearly visible.

In a preferred form, this treatment consists of disposing the plate adjacent a photographic film, with an ultra-violet opaque filter placed in between, and exposing the plate to an ultra-violet light source. The fluid droplets on the plate are thus caused to fluoresce and an image of the droplets is recorded on the film.

By thus ascertaining the exact pattern of back-streaming which is occurring, appropriate adjustments and corrections may be made on the pump with more surety and many trial and error corrections may be eliminated.

Accordingly, it is an object of the present invention to facilitate the operation of vacuum pumps of the diffusion type.

It is another object of the invention to facilitate the suppression of errant vapor streams in diffusion pumps.

It is another object of the invention to provide a method and means for precisely measuring the migration of pump ing fluid from a diffusion pump housing to the chamber being evacuated.

It is a further object of the invention to provide a method and means for locating the sources of back-streaming fluid in a diffusion pump by determining the pattern of vapor flow in a cross-section of the pump throat.

It is still another object of the invention to provide a method and apparatus for determining the pattern of vapor flow in the cross-section of a diffusion pump throat which method does not significantly interfere with the operation of the pump.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be best understood with reference to the following specification taken in conjunction with the accompanying drawing of which:

FIGURE 1 is a broken out elevation view of a diffusion pump showing elements of the present invention associated therewith,

FIGURE 2 is an elevation section view of an airlock mechanism associated with the pump,

FIGURE 3 is a plan section view of the apparatus of FIGURE 2 taken along line 3-3 thereof, and

FIGURE 4 is a broken out perspective view illustrating the processing of a back-streaming indicator plate after removal from the apparatus of FIGURES 1 to 3.

Referring now to the drawing and more particularly to FIGURE 1 thereof there is shown a tank 11 which is to be evacuated and which has a flanged cylindrical port 12 projecting downwardly for coupling with the upper end of a diffusion vacuum pump 13. A flat rectangular air lock assembly 14, to be hereinafter described, is secured between the port 12 and pump 13, the junctures between the port, airlock and pump being hermetically sealed.

Pump 13 is of conventional design and thus includes a cylindrical housing or barrel 16 which is closed at the lower end by a baseplate 17 having heating coils 18 embedded therein. A pool of fluid 19, generally oil or mercury, is contained within barrel 16 over baseplate 17, the fluid being vaporizable by heating from the baseplate.

To form downwardly directed vapor jets within barrel 16, an annular stack assembly 21 is disposed coaxially therein. Stack 21 in this instance is of the type forming three vertically spaced jets and thus includes a lower tubulation 22, intermediate tubulation 23 and upper tubulation 24, of progressively diminishing diameter. A dome shaped cap 26 is mounted over the upper tubulation 24. The lower ends of cap 26, upper tubulation 24 and intermediate tubulation 23 are each outwardly flared to a diameter larger than that of the adjacent end of the next lower tubulation thereby forming three annular vertically spaced vapor jet nozzles 27.

The lower end of the lowermost stack tubulation 22 is also outwardly flared to a diameter slightly less than that of the barrel 16, and notches 28 are formed around the lower edge of the tubulation to admit fluid 19 thereto.

An upwardly angled conduit 29 connects with the interior of barrel 16 at a position below the lowermost jet nozzle 27 and above the fluid pool 19 to provide for connection of the diffusion pump exhaust with a suitable mechanical forepump. Such forepump is required in that diffusion pumps require a rough vacuum at the outlet for efiicient operation.

The apparatus as described above is a typical diffusion pump and operates by the entrainment of air molecules from tank 11 in the downwardly directed vapor jets from nozzles 27. The vapor in the jets condenses as it strikes the relatively cool wall of barrel 16 and returns to pool 19 while the entrained air is released beneath the jets and withdrawn through exhaust conduit 29.

It is a characteristic of such pumps that a certain amount of the pumping fluid vapor will be released upwardly towards the tank 11. Such back-streaming vapor tends to raise the pressure within the tank 11 and in practice is a primary factor limiting the degree of vacuum that can be obtained. It is therefore of extreme interest to determine the extent of such back-streaming and the pattern thereof with respect to a cross-section of the pump throat. The pattern is of interest in that it is indicative of the exact source of the back-streaming and therefore facilitates the making of design modifications in new pumps and appropriate corrections and repairs in existing pumps. Thus back-streaming data is desirable both from the standpoint of testing new pump designs and for the routine monitoring of individual operating pumps. In the latter instance, such information provides a means for detecting malfunctions and for indicating appropriate corrections.

The present invention provides for the obtaining of back-streaming information by momentarily introducing a glass plate 31 across the throat of the pump to intercept and condense a portion of the back-streaming vapor. The plate 31 is then removed from the pump and developed, by a process to be hereinafter described, to obtain a visual indication of the pattern and degree of back-streaming.

Referring now to FIGURES 2 and 3 in conjunction with FIGURE 1, plate 31 is inserted into the pump 13, and removed therefrom, by means of the air lock assembly 14. Airlock 14 comprises a flat rectangular base 32 having upwardly directed side walls and forming a shallow rectangular chamber 33. Base 32 is slightly wider than the diameter of the pump barrel 16 and has a length exceeding twice the barrel diameter. A fiat top plate 34 is secured against the upper surface of base member 32 with a vacuum sealing element 35 being disposed therebetween. One end of the base 32 and top plate 34 is positioned between pump barrel 16 and port 12 and each of the members 32 and 34 has a circular opening 36 and 37 respectively which is equal in diameter to the pump barrel so that the airlock does not obstruct the throat of the pump.

To provide for the insertion and withdrawal of the glass plate 31, a narrow shelf 38 is formed along each of the longer edges of the base 32 within chamber 33. A fiat carriage 39 is disposed within the chamber 33 and is provided with wheels 41 which ride on the shelves 38. Carriage 39 has a circular opening 42, bounded by an annular shelf 43, for receiving the glass plate 31 and spring clips 44 are mounted around the rim of opening 42 to clamp the plate therein.

To .provide for translation of the carriage 39 within the air lock chamber 33, an annular vacuum seal housing 46 is mounted at the end of the base 3 2 which is remote from the pump :13 and a longitudinally slidable rod 47 is transpierced through the seal. Carriage 39 is coupled to rod 47 by a pair of angled projections 49 which enter opposite sides of an annular groove 51 on the inner end of the rod. This mode of coupling allows the carriage 39 to be lifted without interference from the rod 47, as will be hereinafter described.

In order that the glass plate 31 may be removed from airlock 14, a circular opening 52 is situated in top plate 34 over the position of the plate when carriage 39 has been fully retracted from the pump 13. A cylinder 53 is secured to top plate 34 around opening 52 and a removable circular cover plate 54 is sealingly fastened against a flange 56 on the upper rim of the cylinder.

At such times as the cover plate 54 is removed for purposes of inserting or withdrawing the glass plate 31, it is necessary that the interior of the pump 13 be sealed from the region around the glass plate. Considering now structure for effecting this result, a circular opening 57 is provided in the airlock baseplate 32 below opening 52 and in coaxial relationship therewith. A tubulation 58 extends downwardly from baseplate 32, forming a continuation of the opening 57, and is closed at the lower end by a circular cap 59 which seats against a flange 61 and seal 62 on the lower end of the tubula tion. A rotatable rod 63 extends along the axis of tubulation 58 and is threadably engaged in an axial passage 64 in cap 59 so that rotation of the rod, by means of a handle 66 on the lower end thereof, will move the rod in a vertical direction.

A circular disc 67 is disposed beneath carriage 39 and is centered over opening 57, the disc having a diameter exceeding that of the opening 42 in the carriage. A cylindrical boss 68 is formed at the center of disc 67 and extends downwardly into baseplate opening 57. An angled bracket 69 extends from the underside of boss 68 and is engaged in a groove 71 on the upper end of rod 63. Thus rotation of handle 66 in a first direction acts to raise the disc 67 against the underside of the carriage 39 and subsequently to lift the carriage itself into abutment against the underside of the airlock top plate 34. Rotation of handle 66 in the opposite direction will lower the carriage 39 to its normal position on shelves 38 and will subsequently lower disc 67 an additional amount to permit free lateral movement of the carriage.

To prevent the leakage of air through the threaded passage 64 in cap 59, a cylindrical bellows 70 encloses rod 63 between disc projection 68 and the upper surface of cap 59.

In order that the region around the glass plate 31 will be hermetically sealed from pump 13 when the carriage 39 is elevated as described above, an annular O-ring seal 72 is disposed on the upper surface of the carriage in a groove 73 which encircles opening 42 thereof and a similar seal 74 is disposed in a coaxial groove 76 on the upper surface of disc 67. Such seals are compressed when the carriage 39 is elevated thereby hermetically isolating the region above disc 67 to permit removal of the airlock cover plate 54. A series of small holes surrounding the carriage opening 42 provide for the escape of air trapped between disc 67 and the carriage 39. Since the cover plate 54 cannot readily be lifted while there is vacuum within cylinder 53, a valve 77 is coupled to a port 78 in the wall of the cylinder 53 to admit air thereto just prior to removal of the cover plate. To provide for the pump-out of cylinder 53 prior to lowering of carriage 39, valve 77 has an alternate setting which communicates the cylinder with an exhaust pump 75.

Considering now the initial steps in monitoring backstreaming in the pump 13, and assuming the pump to be operating, handle 66 is rotated to elevate carriage 39 as heretofore described thereby sealing the region above disc 67 from the remainder of chamber 33 and the pump. Following opening of valve 77 to admit air to cylinder 53, cover plate 54 is removed and the glass plate 31 is seated in opening 42 of carriage 39, the glass being fastened in position by spring clips 44, The cover plate 54 is then replaced, cylinder 53 is evacuated by pump 75, and the carriage 39 is lowered by reverse rotation of handle 66.

Carriage 39 may then be traversed within chamber 33, by means of rod 47, to bring the glass plate 31 into position across the throat of pump 13 as shown in FIGURE 1. Under usual operating conditions, the plate 31 is left in the pump for an interval of several seconds. During this time back-streaming vapor will be deposited on the underside of the plate in an amount which is indicative of the degree of back-streaming and with a pattern which will aid in determining the principal sources of the backstreaming vapor.

Plate 31 is then removed from the pump 13 by retract- 7 tion or rod 47 to bring carriage 39 beneath cylinder 53 of the airlock. Handle 66 is then rotated to elevate the carriage 39 thereby sealing the airlock chamber 33. Air is then admitted ,to cylinder 53 by operation of valve 77 and cover plate 54 is lifted to provide access to plate 31.

The pumping fluid deposited on plate 31 may not be sufliciently visible to allow the desired data to be observed directly in which case the plate may be processed to render the pattern clearly visible and to provide a permanent record thereof. Referring now to FIGURE 4, a preferred method for developing the plate is illustrated, the

ethod making use of the fact that the pumping fluid will fluoresce under ultra-violet light whereby the pattern may be recorded on a photographic plate.

To make the exposure, a film plate '79, having an area equal to that of glass plate 31, is disposed in the bottom of a shallow light-tight container 81. A first filter plate 82 is placed over film '79, the filter being of a type which is transparent to the light of fluorescence of the pumping fluid and opaque to ultra-violet light. Glass plate 31 is placed immediately over filter 82 and a second filter plate 83 is placed over the top of the assembly. Filter plate 83 is of a type opaque to ambient visible light and transparent to ultra-violet light.

When the above described elements have been assembled in container 81, the exposure is made by directing an ultra-violet light source 84. toward the top filter 83 after which the film 79 is removed and developed in the conventional manner. The degree and pattern of backstreaming within pump 13 will then be evident on the film.

Other techniques for developing and recording the backstreaming pattern on glass plate 31 may be employed. For example, a small amount of radioactive material may be added to the pumping fluid so that the plate 31 may simply be scanned by any of the forms of radiation counter which record the distribution of radioactivity on the object by displaying the pattern on a cathode-ray-tube screen or other such means.

Thus the invention provides an inexpensive and extremely convenient method for monitoring this important phase of the operation of diffusion pumps whereas previous measurement techniques of back-streaming have involved complicated and time-consuming trial and error methods. It will be apparent that elements of the invention may be readily built into any standard diffusion pump to facilitate obtaining improved operation in any of the various applications to which such pumps are put.

While the invention has been described with respect to a particular embodiment thereof, it will be apparent to those skilled in the art that numerous variations and modifications are possible Within the spirit and scope of the invention and it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a method for determining the pattern of backstreaming vapor in a diffusion pump, the steps comprising:

(a) placing a flat plate across the throat of said pump,

(b) operating said pump for a limited period of time with said plate therein, and

(c) removing said plate from said pump for examination. the umping fluid deposited thereon being indicative of said pattern.

2. In a method for detecting the degree and pattern of back-streaming pumping fluid vapor in a vacuum pump of the diflusion class, the steps comprising:

(a) inserting a flat plate transversely across the throat of said pump,

(b) operating said pump with said plate therein,

(c) removing said plate from said pump after a short interval in which said plate is stationary therein, and

(d) treating said plate to intensify the pattern formed by said pumping fluid deposited thereon.

3. A method for detecting the degree and pattern of back-streaming pumping fluid vapor in a vacuum pump as described in claim 2 wherein said treatment of said plate comprises:

(a) placing a photographic film adjacent said plate,

(b) irradiating said plate with ultra-violet light whereby said photographic film is illuminated by the light of fluorescence of vapor deposited on said plate, and

(c) subsequently developing said film whereby said pattern of back-streaming vapor is visible thereon.

4. A method for observing the intensity and distribution of back-streaming pumping fluid vapor in a diffusion pump com-prising, the steps:

(a) disposing an airlock at the throat of said pump,

(b) inserting a smooth flat plate into said airlock, said plate having a size at least equal to that of the crosssectional area of said pump throat,

(c) translating said plate from a first position in said airlock to a second position across the throat of said pump while said pump is operating,

((1) translating said plate back to said position in said airlock after a short interval during which said plate is stationary in said second position across said pump throat,

(e) removing said plate from said airlock without disturbing the under surface of said plate,

(f) exposing said plate to actinic irradiation whereby pumping vapor deposited thereon is caused to fluoresce, and

(g) photographing the light pattern of vapor fluorescence on said plate.

5. In a method for detecting the degree and distribution of back-streaming pumping fluid vapor in vacuum ump of the diffusion class, the steps comprising:

(a) introducing a small amount of radioactive mate rial to said pumping fluid,

(b) inserting a flat plate transversely across the throat of said pump,

(c) operating said pump for a limited period of time with said plate emplaced therein,

(d) removing said plate from said pump, and

(e) scanning said plate with a radiation counter of the type which displays the distribution of radioactivity in an area being scanned.

6. Apparatus for monitoring back-streaming in the throat of a diffusion pump, comprising:

(a) a flat plate,

(b) an airlock disposed at the wall of said pump at the throat section thereof, said airlock having a selectively sealable opening for receiving said plate therein and for subsequent removal of said plate therefrom, and

(0) means within said airlock traversing said plate to a position Within and transverse to the throat of said pump and for retracting said plate from said pump throat after a short duration thereat'.

7, Apparatus for visually observing the attern of backstreaming pumping fluid vapor from a diffusion pump housing into a vacuum vessel, comprising:

(a) a flat plate having a surface area at least equal to the cross-sectional area of the intake passage of said P p,

(b) an airlock housing secured between said pump and said vacuum vessel and normally providing an unobstructed passage therebetween, said airlock housing having a scalable chamber for receiving said plate in a position to one side of said pump throat and having a passage for traversing said plate from said chamber into an inserted position across the throat of said pump and for retracting said plate therefrom, said scalable chamber in said airlock housing further providing for subsequent removal of said plate at said position at one side of said pump throat, and

(c) an externally operated element extending into said airlock housing for selectively moving said plate into said inserted position and for withdrawing said plate therefrom into said chamber.

8. Apparatus for visually observing the pattern of backstreaming pumping fluid in a diffusion pump as described in claim 7 and comprising the further combination of:

(d) a secondary vacuum pump for exhausting said sealable chamber of said airlock after said plate has been disposed therein, and

(e) a manually operable valve having a first position which couples said secondary vacuum pump to said sealable chamber and having a second position which disconnects said secondary pump therefrom and admits atmospheric pressure to said chamber.

9. Apparatus for monitoring the back-streaming of pumping fluid between a diffusion pump and an associated vacuum vessel, comprising:

(a) a flat plate having an area substantially as large as the cross-sectional area of the throat of said pump,

(b) an airlock housing secured between said pump and said vessel and forming a thin flat chamber transverse to the axis of said pump and extending to one side thereof, said housing having a broad sealable opening at the side of said pump for inserting said flat plate into said chamber and for subsequent removal of said flat plate from said chamber,

() an annular carriage disposed within said chamber and having a central opening for receiving said plate, said carriage being slidable within said chamber from a position adjacent said airlock opening to a position within the throat of said pump, and

((1) means contained within said airlock for hermetically isolating the region around said carriage from said pump when said carriage is at said airlock opening.

10. Apparatus for monitoring the back-streaming of pumping vapor between a diffusion pump and a vacuum vessel associated therewith, comprising:

(a) a plate having a flat surface,

(b) an airlock housing secured between said pump and said vacuum vessel and forming a flat shallow chamber transverse to the axis of said pump and extending to one side thereof, said housing having a large sealable opening in the upper surface at the side of said pump for inserting said flat plate into said chamber and for subsequently removing said flat plate from said chamber,

(c) an annular carriage slidably disposed in said airlock chamber and having a broad central opening for receiving said plate,

(d) a rod sealingly penetrating the end wall of said airlock housing and communicating at one end thereof with said carriage for selectively translating said plate between a first position beneath said airlock opening and a second position within the throat of said pump, and

(e) means contained within said airlock for hermetically isolating the region around said carriage from said pump when said carriage is at said first position.

11. Apparatus for determining the degree and distribution of back-streaming pumping fluid from a diffusion pump housing into a vessel being evacuated, comprising:

(a) a flat plate having a surface area substantially equal to that of the cross-section of the throat of said pump,

(b) a broad shallow housing secured between said pump and said vacuum vessel transverse to the axis of said pump and extending outward to one side thereof, said housing having a large opening at the side of said pump for inserting said flat plate therein and subsequently removing said flat plate therefrom,

(c) a carriage slidably disposed in said housing and having a broad central opening for receiving said plate,

(d) means for moving said carriage to selectively translate said plate between a first position beneath said opening in said housing and a second position within the throat of said pump,

(e) a removable lid sealably disposed on said housing to close said large opening thereof,

(f) closure means hermetically isolating said carriage and said plate at said first position thereof from said throat of said pump, and

(g) a valve communicating with the region of said housing adjacent said large opening thereof, said valve having an air intake position and an air exhaust position whereby the pressure in said region is controllable independent of the pressure in said pump.

12. Apparatus for monitoring the escape of pumping fluid vapor from a diffusion pump into a vessel being evacuated thereby, comprising:

(a) a flat plate having a surface area substantially equal to the cross-section area of the throat of said p p (b) a broad shallow housing secured between the throat of said pump and said vacuum vessel and extending outwardly at one side thereof, said housing having a large opening in the upper surface thereof for receiving said flat plate therein,

(c) a carriage slidably disposed in said housing and having a broad central opening for receiving said flat plate,

(d) means coupled to said carriage to selectively translate said carriage and said plate between a first position beneath said large opening in said housing and a second position within the throat of said pump,

(e) a closure plate disposed in said housing beneath said opening thereof and said carriage,

(f) externally operated means for selectively lifting said closure plate towards said carriage and said opening of said housing, whereby in the first position of said carriage said closure plate may be moved from a lower position freely beneath said carriage to an upper position forcing said carriage against the upper surface of said housing to hermetically seal the region of said large opening in said housing from said pump,

(g) a removable top plate hermetically closing said large opening in said housing, and

(h) means communicating with the region of said housing adjacent said opening thereof for selectively controlling the pressure in said hermetically sealed region.

13. Apparatus as described in claim 12 wherein said flat plate is formed of a transparent substance.

References Cited by the Examiner UNITED STATES PATENTS 2,984,744 5/1961 Lynch et al 25065 3,102,678 9/1963 Levenson 2301(ll 3,176,623 4/1965 Howerton et al 25071 RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

A. L. BIRCH, Assistant Examiner, 

12. APPARATUS FOR MONITORING THE ESCAPE OF PUMPING FLUID VAPOR FROM A DIFFUSION PUMP KNTO A VESSEL BEING EVACUATED THEREBY, COMPRISING: (A) A FLAT PLATE HAVING A SURFACE AREA SUBSTANTIALLY EQUAL TO THE CROSS-SECTION AREA OF THE THROAT OF SAID PUMP, (B) A BROAD SHALLOW HOUSING SECURED BETWEEN THE THROAT OF SAID PUMP AND SAID VACUUM VESSEL AND EXTENDING OUTWARDLY AT ONE SIDE THEREOF, SAID HOUSING HAVING A LARGE OPENING IN THE UPPER SURFACE THEREOF FOR RECEIVING SAID FLAT PLATE THEREIN, (C) A CARRIAGE SLIDABLY DISPOSED IN SAID HOUSING AND HAVING A BROAD CENTRAL OPENING FOR RECEIVING SAID FLAT PLATE, (D) MEANS COUPLED TO SAID CARRIAGE TO SELECTIVELY TRANSLATE SAID CARRIAGE AND SAID PLATE BETWEEN A FIRST POSITION BENEATH SAID LARGE OPENING IN SAID HOUSING AND A SECOND POSITION WITHIN THE THROAT OF SAID PUMP, (E) A CLOSURE PLATE DISPOSED IN SAID HOUSING BENEATH SAID OPENING THEREOF AND SAID CARRIAGE, (F) EXTERNALLY OPERATED MEANS FOR SELECTIVELY LIFTING 